1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly to a method for manufacturing a semiconductor device having recess gates with uniform trenches.
2. Description of Related Art
As DRAM cells have become highly integrated, design rules for the miniaturization of devices have been developed. Thus, the size of cell transistors and the length of transistor channels have decreased. The decrease in the length of the channels generates a short channel effect, which causes a decrease in the threshold voltage, an increase in the leakage current, and deterioration of refresh characteristics. Accordingly, a semiconductor device having recess gates has been proposed, in which the length of the channel is increased without increasing the footprint of the gate by extending the channel downward into the substrate.
FIG. 1 is a sectional view of a conventional semiconductor device having recess gates.
With reference to FIG. 1, trenches 111, having a designated depth from the surface of a semiconductor substrate 100, are formed in the semiconductor substrate 100. The active regions in a cell are restricted by an isolation film 102. Then, a gate stack 112 is disposed in each of the trenches 111, and source/drain impurities are implanted into the semiconductor substrate 100, thus forming channels (A). The gate stack 112 includes a gate dielectric film pattern 104, a semiconductor layer pattern 106, a metal film pattern 108, and a hard mask film pattern 110. Compared with a semiconductor device having flat channels, the semiconductor device having the recess gates has channels of a longer length, thus reducing the short channel effect.
When trenches for recess gates are formed by etching the semiconductor substrate 100, the trenches are designed to have a uniform depth, but may have different depths depending on several process conditions. When the depths of the trenches are not uniform, threshold voltages of the cell regions is not uniformly distributed. Thereby, electrical characteristics of the semiconductor device may deteriorate.